If you have decided to leave his company by dissolving the marriage, it is your decision, you can decide about it based on the prevailing circumstances at your end.
You can file a contested divorce case on the grounds of cruelty.
About child custody, you dont have to be worried about the whatsapp messages or any other social media messages to prove his allegations, because they can be disproved if proper procedure to establish the genuineness of such messages have not been adopted, moreover smooch is not a big offence or an act of adultery to blame a woman as characterless.
This can fought back properly in the trial proceedings.
You keep the child with you and let him file a child custody case and fight on it.
You secure all the evidences and also fight back on merits.